Since a few decades the art of steel cords has known a continuous trend towards simpler constructions comprising fewer and thicker filaments and requiring fewer manufacturing steps.
In this way steel cords comprising multiple strands have been replaced by layered steel cord constructions, i.e. steel cords comprising a core and one or more layers such as a 3+6, a 3+9 and a 3+9+15. Of these layered steel cord constructions a 3+6 construction, such as a 3.times.0.20+6.times.0.35 construction, is a widely used construction.
Further continuing the above-mentioned trend and trying simultaneously to achieve full rubber penetration between the individual steel filaments of the steel cords, attempts have been made to replace this 3.times.0.20+6.times.0.35 construction by a 1+6 construction. The reason is that a 1+6 construction only requires one twisting step in the manufacturing whereas a 3+6 construction requires two twisting steps in the manufacturing. These attempts, however, have been unsuccessful up to now.
Replacement of a 3.times.0.20+6.times.0.35 by a 1.times.0.35+6.times.0.35 failed because the 1+6 suffered from core migration, bad compression behavior and because there was no rubber penetration between the six layer filaments and the core filament.
With a 1.times.0.38+6.times.0.35 construction, such as disclosed in WO-A-89/09305, the problem of core migration was at least partially solved, but the construction still suffered in some cases from an insufficient degree of rubber penetration due to the possible shifting of the filaments in the layer towards each other and thus causing interstices where rubber can no longer penetrate.
Another disadvantage is the use of two different diameters with the resulting danger of confusion of filaments during industrial production and of even a greater risk of core migration in case the thicker filament is located in the layer instead of in the core.
Another attempt has been to propose a 1.times.d.sub.1 +6.times.d.sub.2 construction where one or more of the layer filaments had been plastically deformed in such a way that they obtain a wavy form. The object of this wavy form is to create micro-gaps between the layer filaments and the core filament to allow rubber to penetrate. Experiments, however, have shown that rubber penetration is only sufficient if the amplitude of the wave form becomes excessively large or if the core filament diameter d.sub.1 is substantially larger than the layer filament diameter d.sub.2.
A further attempt has been disclosed in EP-A-0 551 124 and in EP-A-0 619 398. Here the core filament has been subjected to a plastic deformation instead of the layer filaments in order to create the necessary micro-gaps between the layer filaments and the core filament. Here again, experiments have shown that rubber penetration is only sufficient with an excessively large wave amplitude or with a core filament diameter d.sub.1 substantially larger than layer filament diameter d.sub.2.